High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study
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2011 Elsevier Ltd. All rights reserved.
ated inease cn conupgrafuentudge (0% of i
WWTPs in some undeveloped countries. For example, up to 2010,among the 2500WWTPs in China, only 50 ones were designed withanaerobic digestion systems, and only 20% of them were well oper-ated. Low-solid anaerobic digestion was not well applied in Chinamainly due to poor management, unprofessional operation,economical limitation and inadequate planning. However, morethan 80% of the sewage sludge in China has already been dewatered
shima et al. (1999) has suggested a system in which the dewateredsludge discharged from small-scale plants is collected and sent to aplant with an anaerobic digester. They investigated the effect ofmoisture content on anaerobic digestion of dewatered sludge, butthe TS contents of the fed sludge used in their study were below11% (w/w), and a long-term effect was not investigated. Nges andLiu, 2010 investigated the effect of solid retention time (SRT) onanaerobic digestion of dewatered sludge inmesophilic and thermo-philic conditions in long-term experiments, but the TS content offed sludge was below 12%. Up to now, there are few studies related
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Bioresource Technology 104 (2012) 150156
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elsE-mail address: firstname.lastname@example.org (B. Dong).essary stabilization. Thus, it is essential to develop proper treat-ment processes to reduce the amount of sludge. One of the mostwidely used processes is anaerobic digestion or methanisation. Itcan realize sludge stabilization by converting a part of its organicmatter into biogas which is a renewable energy source. This tech-nology has been successfully implemented in the treatment ofagricultural wastes, food wastes, and sewage sludge (Chen et al.,2008).
However, the use of traditional anaerobic digester for low-solidsewage sludge is not always feasible in small-scale WWTPs or
less material handling, and so on (Guendouz et al., 2008).So far, a wide range of organic solids found in municipal, indus-
trial, and agricultural wastes have been investigated as feedstocksin high-solid anaerobic digestion, including food wastes (Choet al., 1995; Lu et al., 2007), agricultural wastes (He et al., 2008; Lis-sens et al., 2004; Mosier et al., 2005; Pang et al., 2008), and organicfraction ofmunicipal solidwastes (OFMSW) (Bolzonella et al., 2006;Forster-Carneiro et al., 2007; Martin et al., 2003; Sans et al., 1995).However, so far, no reports can be found focusing on high-solidanaerobic digestion of sewage sludge with feeding TS of 20%. Fuji-1. Introduction
Volume and mass of sludge generplants (WWTPs) are expected to incrdecade, due to increasing populatioworks, building of new WWTPs andto fulll the more stringent local ethe annual production of sewage slhas reached almost 3000 tons, and 80960-8524/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.10.090waste water treatmentontinuously in the nextnected to sewage net-ding of existing plantsregulations. In China,80% moisture content)t has not obtained nec-
before further disposal or treatment, whichmakes it favorable to becentralized processed. Hence, high-solid anaerobic digestion couldbe one viable option to solve the sludge disposal problems inundeveloped countries like China or in small-scale WWTPs.High-solid anaerobic digestion is usually characterized by a highTS content of the feedstocks, typically greater than 15% (w/w)(Rapport et al., 2008) and has been claimed to be advantageous overtraditional low-solid anaerobic digestion for several reasons, suchas smaller reactor volume, lower energy requirements for heating,High-solidMesophilic
higher OLR (46 times as high) and obtain similar methane yield and VS reduction as conventionallow-solid system at the same SRT, thus reach much higher volumetric methane production rate.High-solid anaerobic digestion of sewageFeasibility study
Nina Duan, Bin Dong , Bing Wu, Xiaohu DaiNational Engineering Research Center for Urban Pollution Control, School of Environment
a r t i c l e i n f o
Article history:Received 10 September 2011Received in revised form 22 October 2011Accepted 25 October 2011Available online 3 November 2011
Keywords:Anaerobic digestionSewage sludge
a b s t r a c t
Feasibility of high-solid anastirred tank reactors at 35retention time (SRT) and toExperimental results show600 mg l1, high-solid anaemoderate and signicant i600 and 600 to 800 mg l1
nicant ammonia inhibiti
journal homepage: www.ll rights reserved.ludge under mesophilic conditions:
cience and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China
bic digestion of sewage sludge was investigated in single-stage completelyC. System stability and the effect of organic loading rate (OLR), sludgesolid (TS) content on the performance of high-solid system was examined.that, with the concentration of free ammonia nitrogen (FAN) lower thanic digestion of sewage sludge could maintain satisfactory stability. Slight,ition was found with FAN concentration ranging from 250 to 400, 400 tospectively. The VFA/TA criteria could not foresee system instability in sig-system by its traditional ratio grades. High-solid system could support
evier .com/locate /bior tech
to high-solid anaerobic digestion of sewage sludge with TS above12% in a long-term operation. In this study, high-solid mesophilicdigestion of sewage sludge (feeding TS 20%) was investigated usingsingle-stage completely mixed reactors (semi-continuously oper-ated), the performance of which were compared with reactors fedby TS of 10% and 15%. The effect of OLR, SRT and TS content wasexamined, with special attention paid to ammonia inhibition andsystem stability in high-solid state. The aim was to investigate
6890N, CA, USA) with a thermal conductivity detector equipped
10,000 rpm for 10 min. Then the supernatant was passed througha microber lter (0.45 lm) and the ltrate was acidied by formicacid to adjust the pH to approximately 2.0 before VFA was ana-lyzed by a GC (Agilent Technologies 6890N, CA, USA) with ameionization detector. TS, VS, TA and TAN were determined accordingto standard methods (APHA, 1995). FAN concentration was calcu-lated in the same way as described by stergaard (1985). The deg-radation or removal level based on VS (i.e. VS reduction) was
tion), indicating better buffering capacity and successful recovery
N. Duan et al. / Bioresource Technology 104 (2012) 150156 151with Hayseq Q mesh and Molsieve 5A columns. To analyze VFAs,the sludge samples from the reactors were centrifuged at
Table 1Characteristics of the substrate and inoculums (TS and VS values reported are averag
Parameters DSa 1 (days 155) DS 2 (days 56115) DS 3 (day
pH 7.4 7.4 7.5TS (%, w/w) 20.45 22.56 23.08VS/TS (%) 60.09 60.29 52.48TAN (mg l1) 783 694 801the stability of high-solid anaerobic digestion of sewage sludgeand to evaluate its performance.
2.1. Substrates and inoculums
Dewatered sewage sludge from Anting WWTP (Shanghai, Chi-na) was used as substrate for the present study. The sludge was ob-tained by collecting primary and excess sludge and dewatered withthe aid of a high-molecular occulant based on polyacrylamide.The total solids (TS) of the dewatered sludge ranged from 19% to23% (w/w) and volatile solids (VS) accounted for 5060% of TS.The mesophilic seed sludge collected from an anaerobic digesterat Bailonggang WWTP (Shanghai, China) had TS of 2.3% (w/w)and VS was 56.5% of TS. Characteristics of dewatered sludge andinoculums are listed in Table 1. The collected sludge was storedat 4 C and heated to 35 C before everyday feeding.
2.2. Reactors and operation
Three identical reactors (numbered R1, R2 and R3), with liquidworking volume of 6.0 l, were equipped with helix-type stirrers,which were set at a rate of 60 rpm (rotations per minute) with10 min stirring and 10 min break continuously. Volumes of pro-duced biogas were measured by wet gas meters every day.
On the rst day of the experiments, 6.0 l seed sludge was addedto each reactor, which was operated semi-continuously (once-a-day draw-off and feeding) at 35 1 C. During the start-up period,when the TS concentration of the substrate in the reactor did notreach its designed level, the OLR was increased stepwise withdewatered sludge as feedstock. Once the TS of the substrate in eachreactor approached its designed level, the feeding sludge was di-luted to its designed TS level (10%, 15% and 20%, respectively) withde-ionized water before feeding.
2.3. Analytical methods
Biogas and substrate samples of the reactors were taken twice aweek and analyzed for methane content, pH, TS, VS, volatile fattyacid (VFA), total alkalinity (TA), total ammonianitrogen (TAN)and free ammonianitrogen (FAN). Methane content of the biogaswas measured by a gas chromatograph (GC) (Agilent TechnologiesFAN (mg l1) 26 23 33
a Dewatered sludge.of the systems.It should be noticed that, although the system was inoculated
by digestate from traditional low-solid digester treating sewagesludge and experienced intense unstable state (VFA/TA 0.8-1.1)by overloading, it could still successfully recover in continuousoperation at OLR 2.0 kg VS m3 d3, which was still higher thanthe regular OLR applied in low-solid digesters treating sewagesludge. The results indicated that, as long as pH was not dramati-cally inuenced and was kept in favorable range for methanogens,previous high VFA/TA state was quite recoverable in continuous
6141) DS 4 (days 142160) DS 5 (days 161200) Inoculums
7.4 7.6 7.819.72 22.35 2.3152.47 51.17 56.51
752 739 321calculated by the same equation as reported by Koch et al.(2009), assuming that the mass of undegradable material (inor-ganic fraction) is constant:
VSreduction 1 VSdigestate 1 VSfeed VSfeed 1 VSdigestate %; 1
where VSdigestate is the loss on ignition of digestate (% of TS) andVSfeed is the loss on ignition of feeding sludge (% of TS).
3. Results and discussion
3.1. Start-up performances
Figs. 13 show the performance data of R1, R2 and R3, whichwere fed with dewatered sludge until each reached its designedTS level (on day 30, 68 and 85, respectively). Then the feedingsludge was diluted to its designed TS level (10%, 15% and 20%,respectively) with de-ionized water before feeding. In the rst17 days, each reactor was operated at an OLR of 4.1 kg VS m3 d3,which was much higher than that in traditional low-solid digestionsystem for sewage sludge (Metcalf and Eddy, 2003; Qasim, 1999;Turovskiy and Mathai, 2006). Signicant VFA accumulation andsharp increases of VFA/TA ratio were observed, accompanied bydecreasing biogas production (days 520) and VS reduction (days1220), which indicated low stability at high starting OLR, proba-bly due to the small amount of biomass and its poor acclimationto the substrate at the beginning.
As OLR reducing to 2.0 kg VS m3 d3, methane yield of eachreactor showed increasing trend with great uctuation, probablydue to the conversion of accumulative VFAs to biogas. From day20 to the end of the start-up period (day 30, 68 and 85, respec-tively), the TA, VS reduction and TAN concentration in each reactorshowed increasing trend with TS increasing (the drop of TAN con-centration on day 20 may be caused by the initial drop of VS reduc-25 38 26
15.0 OLR SRT TS
/ m3 .
152 N. Duan et al. / Bioresource Toperation, because VFA accumulation would help providing suf-cient substrate for multiplication of methanogens to same extent.
3.2. Ammonia inhibition and system stability
It should be noticed that, at the end of the start-up period of R2,the VFA concentration was still relatively higher than normal level.At the end of the start-up period of R3, the VFA concentrationraised rapidly to 4500 mg l1 at day 85, with a dramatic decreaseof methane yield. That may probably due to the decreasing meth-anogenic activity caused by ammonia inhibition (De Baere et al.,1984; Hansen et al., 1998; Lay et al., 1997). High VFA levels and al-most steady VS reduction in this period indicated that the acido-genic activity was not inuenced signicantly, which was inaccordance with the report of Koster and Lettinga (1988).
To relieve the dual inhibition caused by ammonia and high VFA,the OLR of Reactor 3 was reduced to 2.0 kg VS m3 d3 after day 85.Since free ammonia has been suggested to be the actual toxicagent, the concentrations of both TAN and FAN are shown in Figs.13.
It was obvious that methanogens were not evidently inuencedwith TAN 10002500 mg l1 and FAN lower than 250 mg l1
(Fig. 1), since no VFA accumulation and deterioration in biogasproduction (the sharp decrease on day 116 was due to the VS/TSchange in feeding sludge) was shown. Evident inuence was
0 20 40 60 80 1000
Fig. 1. Performance data of0
FAN VFA TA TAN pH
ology 104 (2012) 150156exhibited at FAN 500600 mg l1 (Figs. 2 and 3), which led to slightaccumulation of VFAs. Signicant inhibition occurred with FAN600800 mg l1 (Fig. 3), leading to intense VFA accumulation anddramatic drop of biogas production.
The results suggested that, in high-solid anaerobic digestion ofsewage sludge, free ammonia concentration was an important fac-tor inuencing the stability of the system. Taken into considerationof VFA accumulation and biogas production as the main factors toevaluate the real stability degree (VFA accumulation without dra-matic drop of biogas production was considered steady state, anddramatic drop of biogas production was considered a sign of greatinstability), the inhibitory effect of free ammonia can be classiedto several degrees as listed in Table 2.
As shown in Table 2, in a high-solid anaerobic system, whereboth free ammonia and VFAwere important factors affecting meth-anogenic activity, the parameter of VFA/TA ratio was not feasible toevaluate the stability of the system. In the situation of slight inhi-bition, system stability was affected mainly by VFA concentration,and successful recoverywas proved at high OLR (2.0 kg VS m3 d3)with VFA accumulation up to 10 g l1 (see start-up performance)when the VFA/TA ratio was near 1.0. Once full recovery was ob-tained from overloading, the VFA/TA ratio would decrease to0.030.12 in the slight inhibition situation. In the situation ofmoderate inhibition, slight VFA accumulation existed as a resultof decreased methanogenic activity caused by ammonia inhibition,
120 140 160 180 200
VS reduction methane yield (Y) VFA/TA
( l /
d ), V
R1 (feeding TS of 10%).
O S T
TAN pH 12
/ m3 .
N. Duan et al. / Bioresource Tbut the VFA/TA ratio was still below 0.2 and the system was in ainhibited steady state. As the inhibitory effect reached signicantdegree, both FAN and VFA concentration rose to higher levels, lead-ing to great instability of the system. However, the VFA/TA valuewas still below 0.3, which was used to indicate good stability in tra-ditional low-solid digestion systems (Switzenbaum et al., 1990).The result suggested that, in signicant ammonia inhibited system,VFA/TA criteria can no longer foresee system instabilities, at leastby its traditional ratio grades.
The reasons are: in traditional low-solid digestion system, sys-tem instability comes mainly from the decreasing methanogenicactivity caused by accumulated VFAs and the drop of pH it induced.Since the VFA/TA ratio, where VFA concentration represents the fac-tor inuencing system stability and TA concentration representsthe buffering capacity, can properly reect the result of the interac-tion between them, it is then able to evaluate system stability intraditional low-solid digestion system.While in ammonia inhibitedsystem, system instability comes from two aspects, one of which isammonia inhibition, the other is the same as that in low-soliddigestion system. When FAN concentration increased, it contrib-utes more TA (see Eq. (2)) while increases VFA concentrations(within specic range of FAN concentrations, VFA accumulates tocertain level, then causes a decrease of pH, which reduces FAN con-centration, and the system then can reach a new steady state). VFA/TA ratio could not properly reect the stability caused by ammonia,
0 20 40 60 80 100048
Fig. 2. Performance data of4
FAN VFA TA
ology 104 (2012) 150156 153thus could no longer accurately foresee system stability in high-so-lid system, where ammonia inhibition showed more signicantinuence than the other aspects.
NH3 H2O CO2 ! NH4 HCO3 2
3.3. Effect of OLR and SRT
After start-up, all of the three reactors ran at their designedfeeding TS levels; hence their performance parameters at certainOLR or SRT were clearly demonstrated. Table 3 indicates the aver-age performance data of each reactor at certain OLR or SRT afterreaching steady state. Since some of the operational conditionsdid not last more than one SRT, the performance data in the laterperiod of each operational condition was chosen for calculationof an average value. It should be mentioned that, in the rst115 days, the VS/TS (%) of the feeding sludge was about 60%, butfrom day 116 the VS/TS of the feeding sludge used was about52%, due to the change in waste water quality of the WWTP. Thesudden change of VS/TS led to measurement errors in VS reductionsince the VS of the digestate was higher than its real value in thefollowing one or two SRTs of each reactor.
The performance of high-solid system with feeding TS 20% canbe characterized by comparisons as discussed below:
120 140 160 180 200
VS reduction methane yield (Y) VFA/TA
Y ( l
R2 (feeding TS of 15%).
/ m3 .
154 N. Duan et al. / Bioresource Ti. Performance comparison with other reports: The performancedata presented in Table 3 exhibited that with an increase ofOLR or decrease of SRT, the pH value, CH4 content, VS reduc-tion and methane yield showed decreasing trend, while thevolumetric methane production rate increased. The perfor-mance values of the three reactors on VS reduction andmethane yield were relatively lower than that in traditionallow-solid digestion of sewage sludge (Metcalf and Eddy,2003; Qasim, 1999; Turovskiy and Mathai, 2006) (methaneyield 0:3 0:5 l CH4 g1 VS1added d1, VS reduction 5065% atOLR 0.641.6 kg VS m3 d3, HRT 2060 d) and thosereported by some other researchers (De la Rubia et al.,2006; Fujishima et al., 1999; Nges and Liu, 2010), whichmay attributed to the differences in substrate degradabilityinstead of TS content. As is shown in Table 3, VS reduction
900 FAN VFA TA TAN pH
0 20 40 60 80 100048
Fig. 3. Performance data of
Table 2Inhibition description with different ranges of FAN concentration.
Inhibition description FAN (mg l1) TAN (mg l1)
Slight or no
echn0:19 l CH4 g1 VS1added d
1, much higher than that in R1. Itsuggested that at the same OLR, high-solid anaerobic diges-tion could support longer SRT, thus higher VS reduction andmethane yield could be obtained. In addition, the compari-son proved that with lower FAN concentration, high-solidsystem could obtain better performance.
iii. Performance comparison at the same (or similar) SRT: The datain R1 and R2 with a SRT of 30 d showed that at feeding TS of
Table 3Performance data of the three reactors after start-up.
Reactor Feeding VS/TS (%) OLR (kg VS m3 d3) SRT (d) pH V
R1 feeding TS 10% 60 2.0 30.0 8.0 02.4 25.0 7.9 03.0 20.0 7.8 03.5 17.1 7.8 0
52 4.0 13.1 7.6 05.0 10.5 7.6 06.0 8.7 7.3 0
51 8.5 6.0 7.4 012.8 4.0 7.3 1
R2 feeding TS 15% 60 3.0 30.0 8.2 13.5 25.7 8.1 0
52 4.0 19.6 8.0 04.5 17.5 7.8 05.0 15.7 7.6 0
51 6.4 12.0 7.8 08.5 9.0 7.7 0
R3 feeding TS 20% 60 2.0 59.1 8.3 23.0 40.0 8.3 3
52 3.0 24.8 8.2 24.0 26.1 8.1 15.0 20.7 7.9 0
51 6.8 15.0 8.0 08.5 12.0 7.9 0
a VS reduction.b Volumetric methane production rate.
N. Duan et al. / Bioresource T15%, values of VS reduction and methane yield were slightlylower than that at feeding TS of 10% at the same SRT, but thevolumetric methane production rate was much higher,which was due to the digestion at higher feeding TS receivedhigher OLR. Comparisons between R2 and R3 at about 20-day SRT (19.6 d for R2 and 20.7 d for R3) and 12-day SRTshowed similar results. The results suggested that, anaerobicdigestion at higher TS content could support higher OLR andobtain much higher volumetric methane production ratewithout (or with only slight) decline in VS reduction andmethane yield.
It can be calculated that, if operated at the same SRT, high-solidanaerobic digestion system could receive an OLR 46 times as highas that of traditional low-solid anaerobic digestion, with anassumption that the TS of high-solid and low-solid feeding sludgewas 20% and 35% respectively.
The results demonstrated that, with FAN concentration lowerthan 600 mg l1, high-solid anaerobic digestion of sewage sludgeunder mesophilic conditions was feasible and highly practical inlarge-scale application. Since ammonia was produced by the bio-logical degradation of the nitrogenous matter, mostly in the formof proteins, the protein content, VS reduction, as well as the feed-ing TS of the sludge together decided the total ammonia concentra-tion in anaerobic digesters. That means, to avoid high TANconcentration in the digester, high-solid anaerobic digestion ismore suitable to be applied in feeding sludge with lower proteincontent and operated at high OLRs (short SRTs). In addition, sincetemperature and pH signicantly inuence the free ammoniaconcentration, high-solid anaerobic digestion is more suitable toHigh-solid anaerobic digestion treating dewatered sludge (feed-ing TS 20%) was approved feasible in single-stage completely-be applied under mesophilic conditions and operated at low pHvalue (again short SRT was favorable).
(g l1) CH4 (vol.%) VSra (%) Y l CH4 g1 VS1added d1 MPRb (l CH4 l1 d1)
72.2 41.7 0.27 0.5571.0 37.6 0.24 0.5667.9 33.3 0.23 0.6867.7 34.7 0.24 0.8767.5 23.0 0.18 0.7164.7 23.6 0.18 0.9063.6 22.4 0.16 0.9964.5 20.5 0.15 1.2561.2 18.6 0.12 1.49
68.0 39.2 0.25 0.7566.8 37.6 0.25 0.8765.8 21.7 0.19 0.7865.9 24.5 0.22 1.0066.3 28.2 0.20 1.0166.6 30.0 0.19 1.2266.2 28.1 0.19 1.61
66.5 40.4 0.24 0.4965.9 38.7 0.22 0.6765.8 19.8 0.21 0.6365.6 23.6 0.20 0.8164.1 28.6 0.18 0.9265.6 29.7 0.18 1.2464.9 29.0 0.19 1.63
ology 104 (2012) 150156 155mixed semi-continuous reactors under mesophilic conditions.FAN concentration higher than 600 mg l1 was the main factorinuencing system stability. The VFA/TA criteria could not foreseesystem instability in signicant ammonia inhibition system by itstraditional ratio grades. High-solid system could support higherOLR (46 times as high) and obtain similar methane yield and VSreduction as conventional low-solid system at the same SRT, thusreach much higher volumetric methane production rate.
This work was nancially supported by the National Key Tech-nologies R&D Program of China (2010BAC67B04) and the key pro-jects of National Water Pollution Control and Management ofChina (2011ZX07316-004).
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156 N. Duan et al. / Bioresource Technology 104 (2012) 150156
High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study1 Introduction2 Methods2.1 Substrates and inoculums2.2 Reactors and operation2.3 Analytical methods
3 Results and discussion3.1 Start-up performances3.2 Ammonia inhibition and system stability3.3 Effect of OLR and SRT